The In Band On Channel (IBOC) digital audio broadcasting system proposed for usage in the USA places new requirements on broadcast transmitters. Foremost among these is linearity, due to the fact IBOC digital audio modulation has amplitude components. The depth of this amplitude modulation contributes significantly to the cost of transmitter design, as it is necessary to provide adequate headroom and linearity over the entire range of amplitude. Accordingly, it is desirable to reduce this range, a process referred to as crest factor reduction.
A digitally modulated signal is required to arrive at certain vector states (e.g., signal envelope amplitude and phase) at various digital sampling times within the signal. To the degree that the signal deviates from these states, symbol detection errors can occur at an associated receiver. The extent of the crest factor reduction is generally limited by the expected tolerance of associated receivers for the distortion of the vector states. The distortion can be caused by direct compression at the symbol times, or indirectly as a result of compressing the signal between sampling times.
In one example, a hybrid signal can include a frequency modulated analog signal and a digital component. The digital signal transitions between a plurality of possible vector states of the digital component between discrete sample times associated with the hybrid signal, such that the digital signal assumes one of the plurality of vector states at each sample time. It will be appreciated that during these transitions, the hybrid signal can assume amplitudes significantly greater than its amplitude at the individual vector states. This results in an increased crest factor (i.e., Peak-to-average power ratio) of the signal, which is undesirable. Previous attempts to reduce the crest factor of a hybrid signal have caused distortion of the vector states of the signal, resulting in signal errors. Accordingly, the extent of crest factor reduction possible for a given signal is limited by the amount of distortion that is allowable for a given application, as well as by spectral regrowth constraints.